Olefin polymerization, especially ethylene polymerization, can benefit from the addition of longer-chain comonomers, such as 1-hexene, and 1-octene, to produce linear low density polyethylene (LLDPE). LLDPE produced from 1-hexene and 1-octene accounts for a large percentage of the polyethylene resin market. In general, polyethylene plants buy hexene and octene, which are produced in separate plants that typically produce a range of even-numbered alpha olefins from ethylene. It can be expensive to purchase these materials, and they add to the complexity of storage and handling. An attractive alternative is to make the comonomer directly from the ethylene, if this can be done cleanly and economically. It would be perhaps most economical to do this in-situ in the polymerization reactor by altering the catalyst, however this is very difficult.
The review article “Advances in selective ethylene trimerisation—a critical review” by Dixon et al. (J. Organometallic Chemistry 689 (2004) 3641-3668), herein incorporated by reference in its entirety, describes many different catalysts for trimerization. These catalyst systems contain chromium, and with particular ligands, such as aromatic species (e.g. pyrrolyl) or multidentate heteratomic species. The chromium catalysts are typically activated by alkylaluminum and/or alkylaluminoxane activators. The article also describes group 4 and 5 early transition metals, such as Zr, V, Ta and Ti, and group 8 late transition metals, such as Ni, for showing some activity in trimerization.
Phillips has developed and patented chromium-based catalysts that are selective towards making 1-hexene from ethylene. The major byproduct appears to be 1-decene. SRI Consulting PEP Review 95-1-8 entitled “1-Hexene From Ethylene By the Phillips Trimerization Technology,” available on-line at http://www.sriconsulting.com/PEP/Reports/Phase—95/RW95-1-8/RW95-1-8.html, herein incorporated by reference in its entirety, describes the Phillips standalone process for making 1-hexene based on Phillips trimerization technology. In this process, ethylene and a homogeneous catalyst in a solvent are fed to a reactor. The reactor is a stirred tank with heat removal coils. This reactor operates at 115 deg. C. and 49 kg/cm2 (˜700 psia), and converts about 75% of the ethylene fed. This reactor is 42,300 gal (5655 ft3). A spare reactor is provided, since waxy buildup on the cooling coils may necessitate lengthy shutdowns for cleaning. The feed is approximately 29,000 lb/hr cyclohexane solvent (with catalyst) plus 36,000 lb/hr ethylene (27,000 fresh feed and 9,000 recycle). It is estimated that the resident time in the reactor is on average 4 to 5 hours. Selectivity in the Phillips process by weight is about 93% to 1-hexene, 1% to other C6s, 1% to octenes, and 5% to decenes. The effluent from the reactor is contacted with octanol to kill the catalyst from further reaction. The effluent then goes to an ethylene column, where unconverted ethylene is taken overhead and recycled to the reactor. Because ethylene is so volatile, an expensive cryogenic column must be used. Four more distillation columns follow to remove hexene, cyclohexane solvent, octene, and decene. Some of these are run under vacuum, which again makes for expensive hardware and operations. The bottoms from the decene tower is a small stream containing mainly octanol and deactivated catalyst. This stream is treated with caustic and then with acid to remove the catalyst by precipitation and by solution in an aqueous phase, which is separated from the organic phase containing the octanol. Octanol may then be recycled.
U.S. Pat. No. 5,382,738 to Reagen et al., herein incorporated by reference in its entirety, discloses catalyst systems comprising inorganic oxides, modified with a metal alkyl and an unsaturated hydrocarbon, which can be used to support a metal source, such as, for example, chromium, and a pyrrole-containing compound. The resultant catalyst systems can be used to oligomerize and/or trimerize olefins.
U.S. Pat. No. 5,451,645 to Reagen et al., herein incorporated by reference in its entirety, discloses novel chromium-containing compounds prepared by forming a mixture of a chromium salt, a metal amide, and an ether. These novel chromium-containing, or chromium pyrrolide compounds, with a metal alkyl and an unsaturated hydrocarbon, can be used as a cocatalyst system in the presence of an olefin polymerization catalyst system to produce a comonomer in-situ.
U.S. Pat. No. 5,523,507 to Regen et al., herein incorporated by reference in its entirety, discloses novel chromium-containing compounds prepared by forming a mixture of the chromium salt, a metal amide, and an ether either supported or unsupported. These novel chromium-containing compounds are activated by non-hydrolyzed alkyl aluminum compound and a Lewis acid.
U.S. Pat. No. 5,543,375 to Lashier et al., herein incorporated by reference in its entirety, discloses a process to stabilize and/or reactivate an olefin production catalyst system which comprises contacting an olefin production catalyst system, either before or after use, with an aromatic compound, but prior to contacting the system with a reactant.
U.S. Pat. No. 5,563,312 to Knudsen et al., herein incorporated by reference in its entirety, discloses a process to stabilize and/or reactivate an olefin production catalyst system which comprises contacting an olefin production catalyst system, either before or after use, with an aromatic compound.
U.S. Pat. No. 5,859,303 to Lashier, herein incorporated by reference in its entirety, discloses a process in which the solvent is the product of the olefin oligomerization process. This novel process uses a catalyst essentially comprising a chromium compound or chromium salt, a pyrrole-containing compound, and an alkyl compound.
European Pat. No. 0 668 106 to Freeman et al., herein incorporated by reference in its entirety, discloses a process which will effectively deactivate, inhibit, and/or “kill” an olefin production catalyst, and halt polymer production in an olefin production process. It further provides for a process which can remove an olefin production catalyst from the product stream, and recover catalyst by-products for recycle, and/or recovery.
PCT publication WO 99/19280A1 to Woodard et al., herein incorporated by reference in its entirety, discloses a process in which olefins are trimerized in the presence of a catalyst system comprising a chromium source, a pyrrole containing compound and a metal alkyl. The process is preformed in a reactor and provides for a separator for collection of the desired products.
PCT publications WO 2004/056478 to Blann et al. and WO 2004/056479 to Blann et al., both hereby incorporated by reference in their entirety, disclose processes and catalysts to prepare an olefinic stream with more than 30% of 1-octene. The catalysts for this system are those that contain chromium or a chromium salt and a heteroatomic ligand
A need exists for an improved process to generate comonomer in a pre-reactor immediately before the polymerization reactor without isolation of the comonomer. More particularly, a need exists for a reaction/separation process to generate 1-hexene from ethylene immediately before the LLDPE polymerization reactor with no isolation or storage of the hexene produced.